1. Field of the Invention
The present invention relates to a method for manufacturing a composite substrate in which a functional material thin film separated from a substrate composed of a functional material, such as a piezoelectric, pyroelectric, or ferroelectric material, is bonded to the principal surface of a supporting substrate.
2. Description of the Related Art
In recent years, devices using a thin film composed of a functional material have been developed. In the case where a piezoelectric material is used as the functional material, a method may be employed in which a thin film is formed by depositing a piezoelectric material by sputtering, CVD, or the like, or a method may be employed in which a thin film is formed by grinding of a single-crystalline substrate (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-228319).
One example of the device using a piezoelectric thin film is a film bulk acoustic resonator (FBAR) in which a thin film having electrodes provided on the upper and lower surfaces thereof is arranged above a space provided on a supporting substrate so as to be vibrated (for example, refer to International Publication No. WO2005/060091). In the method for manufacturing the FBAR, a thin film is bonded onto a supporting substrate provided with a sacrificial layer, and the sacrificial layer is removed by etching to form a space for oscillation. In the etching process, a pattern, e.g., a via hole, is formed in the thin film to expose the sacrificial layer so that an etchant can reach the sacrificial layer.
Furthermore, a method may be employed in which a release layer is formed by hydrogen ion implantation at a depth of several microns in a crystalline substance of a functional material, and after the crystalline substance is bonded to a supporting substrate, the crystalline substance is cut by heat treatment to form a thin film (for example, refer to Japanese Unexamined Patent Application Publication No. 11-307472 and Japanese Unexamined Patent Application Publication No. 2002-534886). In the method for manufacturing the thin film, the thin film is bonded onto the entire surface of the supporting substrate. Consequently, it is necessary to form a via hole in the thin film in order to extend an electrode from the lower surface to the upper surface of the thin film, or the like.
As described above, in order to form a thin film composed of a functional material by patterning on a supporting substrate, conventionally, a method has been employed in which, after the thin film is bonded to the supporting substrate, the thin film is etched to partially remove the functional material.
However, there are many problems to be solved in a thin film composed of a functional material. For example, it takes too much time to form a pattern because the functional material itself is difficult to etch, and an electrode and a supporting substrate under the functional material are damaged. If it takes time to etch, it becomes a problem that the functional material is damaged and sticking of the resist occurs as a result of etching. In particular, in the case where the functional material is lithium niobate or lithium tantalate, which is a piezoelectric material, lithium compounds produced by etching are chemically stable and inhibit etching, thus greatly affecting the etching time and manufacturing cost. Furthermore, in the case where the functional material is a piezoelectric material, a pyroelectric material, or the like, it becomes a problem that polarization of the substrate may be degraded by etching.